Re: [RFC 0/7] introduce memory hinting API for external process

[Christian Brauner]

On Wed, May 22, 2019 at 06:16:35AM -0700, Daniel Colascione wrote:
> On Wed, May 22, 2019 at 1:22 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> >
> > On Wed, May 22, 2019 at 7:12 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> > >
> > > On Tue, May 21, 2019 at 4:39 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, May 21, 2019 at 01:30:29PM +0200, Christian Brauner wrote:
> > > > > On Tue, May 21, 2019 at 08:05:52PM +0900, Minchan Kim wrote:
> > > > > > On Tue, May 21, 2019 at 10:42:00AM +0200, Christian Brauner wrote:
> > > > > > > On Mon, May 20, 2019 at 12:52:47PM +0900, Minchan Kim wrote:
> > > > > > > > - Background
> > > > > > > >
> > > > > > > > The Android terminology used for forking a new process and starting an app
> > > > > > > > from scratch is a cold start, while resuming an existing app is a hot start.
> > > > > > > > While we continually try to improve the performance of cold starts, hot
> > > > > > > > starts will always be significantly less power hungry as well as faster so
> > > > > > > > we are trying to make hot start more likely than cold start.
> > > > > > > >
> > > > > > > > To increase hot start, Android userspace manages the order that apps should
> > > > > > > > be killed in a process called ActivityManagerService. ActivityManagerService
> > > > > > > > tracks every Android app or service that the user could be interacting with
> > > > > > > > at any time and translates that into a ranked list for lmkd(low memory
> > > > > > > > killer daemon). They are likely to be killed by lmkd if the system has to
> > > > > > > > reclaim memory. In that sense they are similar to entries in any other cache.
> > > > > > > > Those apps are kept alive for opportunistic performance improvements but
> > > > > > > > those performance improvements will vary based on the memory requirements of
> > > > > > > > individual workloads.
> > > > > > > >
> > > > > > > > - Problem
> > > > > > > >
> > > > > > > > Naturally, cached apps were dominant consumers of memory on the system.
> > > > > > > > However, they were not significant consumers of swap even though they are
> > > > > > > > good candidate for swap. Under investigation, swapping out only begins
> > > > > > > > once the low zone watermark is hit and kswapd wakes up, but the overall
> > > > > > > > allocation rate in the system might trip lmkd thresholds and cause a cached
> > > > > > > > process to be killed(we measured performance swapping out vs. zapping the
> > > > > > > > memory by killing a process. Unsurprisingly, zapping is 10x times faster
> > > > > > > > even though we use zram which is much faster than real storage) so kill
> > > > > > > > from lmkd will often satisfy the high zone watermark, resulting in very
> > > > > > > > few pages actually being moved to swap.
> > > > > > > >
> > > > > > > > - Approach
> > > > > > > >
> > > > > > > > The approach we chose was to use a new interface to allow userspace to
> > > > > > > > proactively reclaim entire processes by leveraging platform information.
> > > > > > > > This allowed us to bypass the inaccuracy of the kernelâs LRUs for pages
> > > > > > > > that are known to be cold from userspace and to avoid races with lmkd
> > > > > > > > by reclaiming apps as soon as they entered the cached state. Additionally,
> > > > > > > > it could provide many chances for platform to use much information to
> > > > > > > > optimize memory efficiency.
> > > > > > > >
> > > > > > > > IMHO we should spell it out that this patchset complements MADV_WONTNEED
> > > > > > > > and MADV_FREE by adding non-destructive ways to gain some free memory
> > > > > > > > space. MADV_COLD is similar to MADV_WONTNEED in a way that it hints the
> > > > > > > > kernel that memory region is not currently needed and should be reclaimed
> > > > > > > > immediately; MADV_COOL is similar to MADV_FREE in a way that it hints the
> > > > > > > > kernel that memory region is not currently needed and should be reclaimed
> > > > > > > > when memory pressure rises.
> > > > > > > >
> > > > > > > > To achieve the goal, the patchset introduce two new options for madvise.
> > > > > > > > One is MADV_COOL which will deactive activated pages and the other is
> > > > > > > > MADV_COLD which will reclaim private pages instantly. These new options
> > > > > > > > complement MADV_DONTNEED and MADV_FREE by adding non-destructive ways to
> > > > > > > > gain some free memory space. MADV_COLD is similar to MADV_DONTNEED in a way
> > > > > > > > that it hints the kernel that memory region is not currently needed and
> > > > > > > > should be reclaimed immediately; MADV_COOL is similar to MADV_FREE in a way
> > > > > > > > that it hints the kernel that memory region is not currently needed and
> > > > > > > > should be reclaimed when memory pressure rises.
> > > > > > > >
> > > > > > > > This approach is similar in spirit to madvise(MADV_WONTNEED), but the
> > > > > > > > information required to make the reclaim decision is not known to the app.
> > > > > > > > Instead, it is known to a centralized userspace daemon, and that daemon
> > > > > > > > must be able to initiate reclaim on its own without any app involvement.
> > > > > > > > To solve the concern, this patch introduces new syscall -
> > > > > > > >
> > > > > > > >         struct pr_madvise_param {
> > > > > > > >                 int size;
> > > > > > > >                 const struct iovec *vec;
> > > > > > > >         }
> > > > > > > >
> > > > > > > >         int process_madvise(int pidfd, ssize_t nr_elem, int *behavior,
> > > > > > > >                                 struct pr_madvise_param *restuls,
> > > > > > > >                                 struct pr_madvise_param *ranges,
> > > > > > > >                                 unsigned long flags);
> > > > > > > >
> > > > > > > > The syscall get pidfd to give hints to external process and provides
> > > > > > > > pair of result/ranges vector arguments so that it could give several
> > > > > > > > hints to each address range all at once.
> > > > > > > >
> > > > > > > > I guess others have different ideas about the naming of syscall and options
> > > > > > > > so feel free to suggest better naming.
> > > > > > >
> > > > > > > Yes, all new syscalls making use of pidfds should be named
> > > > > > > pidfd_<action>. So please make this pidfd_madvise.
> > > > > >
> > > > > > I don't have any particular preference but just wondering why pidfd is
> > > > > > so special to have it as prefix of system call name.
> > > > >
> > > > > It's a whole new API to address processes. We already have
> > > > > clone(CLONE_PIDFD) and pidfd_send_signal() as you have seen since you
> > > > > exported pidfd_to_pid(). And we're going to have pidfd_open(). Your
> > > > > syscall works only with pidfds so it's tied to this api as well so it
> > > > > should follow the naming scheme. This also makes life easier for
> > > > > userspace and is consistent.
> > > >
> > > > This is at least my reasoning. I'm not going to make this a whole big
> > > > pedantic argument. If people have really strong feelings about not using
> > > > this prefix then fine. But if syscalls can be grouped together and have
> > > > consistent naming this is always a big plus.
> > >
> > > My hope has been that pidfd use becomes normalized enough that
> > > prefixing "pidfd_" to pidfd-accepting system calls becomes redundant.
> > > We write write(), not fd_write(), right? :-) pidfd_open() makes sense
> > > because the primary purpose of this system call is to operate on a
> > > pidfd, but I think process_madvise() is fine.
> >
> > This madvise syscall just operates on pidfds. It would make sense to
> > name it process_madvise() if were to operate both on pid_t and int pidfd.
> 
> The name of the function ought to encode its purpose, not its
> signature. The system call under discussion operates on processes and
> so should be called "process_madvise". That this system call happens
> to accept a pidfd to identify the process on which it operates is not
> its most interesting aspect of the system call. The argument type
> isn't important enough to spotlight in the permanent name of an API.
> Pidfds are novel now, but they won't be novel in the future.

I'm not going to go into yet another long argument. I prefer pidfd_*.
It's tied to the api, transparent for userspace, and disambiguates it
from process_vm_{read,write}v that both take a pid_t.